In a belt retractor the total range of angular rotation of the belt reel is dictated by the condition of the belt webbing in its fully coiled and fully uncoiled condition. Within this range of angular rotation various relationships exist between the absolute angle of rotation of the belt reel and characteristic conditions of the belt system. Thus, zero angle of rotation (belt webbing fully coiled) is an indication that the belt is not in use. At maximum angle of rotation (belt webbing fully uncoiled) a switching action may be introduced to translate the belt retractor into the automatically blocked condition which is suitable for example for restraining loads or child seats. Between these extreme positions various characteristic conditions may be defined which, where necessary, in conjunction with other parameters, more particularly seating posture, seat occupancy and the like, may be associated with special switching positions in which activating various switching actions is expedient, it being especially desirable for the system to "see" the conditions in which activation of an air bag is to be inhibited, examples of which are the use of child seats, incorrect seated posture (leaning forward) and belt buckled with seat empty. Recognizing such conditions is only possible with adequate reliability when the angle of rotation of the belt reel can be sensed with high accuracy. It is known (e.g. DE 298 20 086.4) to translate the total range of angular rotation of the belt reel by means of a reduction gear unit to a single revolution of an indexing disc. Since, however, the absolute angle of rotation of the belt reel is a multiple of 360.degree., each angle on the indexing disc corresponds to an accordingly greater angular range of the belt reel rotation so that the indexing positions cannot be sensed with the desired accuracy.